Protein dynamics are known to be essential for biological activity, and many proteins undergo large-scale motions as part of their function. Low-frequency, collective (breathing) modes of proteins are believed to be necessary for these large scale motions, and theoretical analyses of a wide variety of proteins have predicted many low-frequency modes. However, direct physical measurement of these modes has proven difficult and the relationships between biological function and low-frequency modes have not been elucidated. The aim of this proposal is to develop a new technology for measuring low-frequency modes. Terahertz (THz) spectroscopy is currently being developed as a technique for answering basic scientific questions in materials science and chemistry. The applicants plan to develop this technology as a tool for measuring low-frequency modes of proteins. THz, time-domain, transmission spectroscopy is a method for measuring absorption coefficients and indices of refraction at low frequency. It is a relatively new technique and neither the materials nor methods for making measurements are well developed. The measurement of protein absorption will pose some challenging problems for technique development. To optimize the likelihood of success, measurements will be made on proteins in which domain movements are known to be important for function. In particular, the applicants will concentrate on lysozyme and calmodulin. Measurements will be made under a range of hydration and temperature conditions to optimize the measurements. The biological importance of low-frequency modes is unlikely to be limited to proteins. Low-frequency modes have been hypothesized to play a role in the dynamics of DNA and RNA in their interactions with proteins. Thus, the technology being proposed for development here is likely to be applicable to an even broader range of systems than the proteins targeted for these first proof-of-principal experiments.